1. Field of the Invention
The present invention relates to a developing method and a developer for electrophotography. More specifically, the present invention relates to a so-called dual-component developer wherein toner particles and carrier particles are mixed, and a developing method for electrophotography employing such developer.
2. Description of the Prior Art
In a method of developing a latent image by a magnetic brush, a mixture of toner particles composed of resin and carbon and carrier particles composed of iron particles, magnetic particles or glass balls is employed as a developer. To be detailed, the both are friction-charged by mixing them, having charges of polarities reverse to each other, and resultingly, by properly selecting a friction charging series of toner particles and carrier particles according to the polarity of a latent image, the latent image is developed as a visible image by making the toner particles adhere electrostatically to a photosensitive drum while neutralizing charges on the latent image.
Developing in electrophotography is a process wherein following F=qE, a toner particle having a charge quantity q is driven by an electric field intensity E corresponding to a latent image on an image carrier surface, and the toner is made to adhere to and is held on the latent image, and the latent image is visualized while the charges on the latent image are neutralized. Actually, the process is complicated by actions of the other forces, but it is unchangeable that this electrostatic Coulombs force F is dominant.
Then, effects on the image quality and on the process of electrophotography were studied with the charge quantity of toner particles, namely, the density of charges on the toner particles employed as a parameter, and the following Table 1 was obtained.
As is obvious from this Table 1, if this quantity of charge on toner particles is large, the charges on the latent image can be neutralized by a small quantity of toner particles. In reverse, if the quantity of charges on the toner particles is small, a large quantity of toner particles are required, and resultingly, the developing density becomes high, but the consumption of toner particles becomes large. Also, if the quantity of charges on toner particles is large, a strong edge effect takes place, and further a so-called hollow phenomenon takes place, and besides, a change in the quantity of charges on toner particles with respect to a change in the density of toner particles is large, and thereby it is very difficult to obtain a constant image quality.
However, if the quantity of charges on toner particles becomes excessively small, an electrostatic attracting force between a toner particle and a carrier particle is reduced, thereby causing the toner particles to scatter in the apparatus in transit of a developer or causing a so-called fogging by adhesion of toner particles onto a non-image part of the latent image surface.
Then, consideration is made on preparing a developer having an intermediate characteristic between the characteristic of a large quantity of charges on toner particles and the characteristic of a small quantity thereof. However, in a developer obtaining such a characteristic by controlling only the characteristic or the value of physical property of the developer, when it is used repeatedly, the characteristic itself changes and resultingly, a so-called spend phenomenon takes place that the developer shifts to the one having either of the characteristic of a large quantity of charges and the characteristic of a small quantity of charges. This means that even a properly prepared developer has a problem of lacking the stability with time of the quantity of charges on the toner particles.
Also, so-called color copying machines roughly includes machines having a monotone multi-color copying function and machines having a full-color copying function. In the former, developing is performed at different developing stations for respective colors by developers containing color toner particles desirably colored, and thereby a monotone multi-color copy is obtained. In the latter, a latent image is developed by developers of respective colors containing respective color toner particles of yellow, magenta and cyan, and they are superposed to obtain a full-color, over-coated color copy.
A large difference of such color toner particles used for color copying machines or the like from the above-described black toner particles is an addition of a dye or a pigment which makes a required coloring in place of a black pigment, carbon black, magnetite or the like.
The conventional color developers which are obtained by mixing color toner particles with carrier particles have different electric characteristics (quantity of charges on toner particles and distribution of quantity of charges) and different physical properties (conveyability and agglomeratability) for the developer of each color depending upon the difference in the added dye or pigment or charge controlling agent. Accordingly, for example, in the case of the electronic copying machine or the like, conditions for the copying process, for example, conditions for charging, transferring, developing and fixing are required to be set for each color in accordance with differences in these characteristics.
Also, imbalances in the developing characteristics and the persistence of image quality take place between the developers of respective colors depending upon the above-mentioned differences in the electric characteristics and the physical characteristics, and therefore it is required to average or equalize the quantities of charges, distributions thereof, effects of the quantity of charges due to change in toner density, fixing properties, image characteristics or particulate characteristics of the developers of respective colors.
For this reason, conventionally, in the case where color developers are used, in designing base resins, colorants or charge controlling agents for color toner particles, the degree of freedom thereof is limited to a great extent, and a closer consideration is required in comparison with the conventional black developers.
This means that the conventional method of developing a latent image by using color developers of respective colors whose main components are color toner particles and carrier toner particles has many practical difficulties due to the above-described problems peculiar to the developers. Such difficulties are present likewise also in the case of such copying machine that obtains a monocolor copy by replacing a developing apparatus for each color.
Therefore, a principle object of the present invention is to provide a developing method for electrophotography and a developer for it which can obtain an electrophotograph of a stable image quality over a long period.
Another object of the present invention is to provide a developing method for electrophotography and color developers for it which are suitable for the color electrophotographing apparatus.
In brief, the present invention is a developer wherein toner particles are mixed with two or more kinds of carrier particles whose charge giving characteristics onto the toner particles differ from one another.
In accordance with the present invention, the distribution of the quantity of charges in the developer is improved by the presence of carrier particles whose charge giving characteristics onto toner particles are different, and thereby an ideal distribution can be achieved. That is to say, in accordance with the present invention, the developed image quality can be improved without complicating the developing apparatus, without requiring any special additional apparatus and also without strictly controlling the developer characteristics of carrier particles and toner particles. Also, spreading with time of the distribution of quantities of charges on toner particles, that is, deterioration of the developer can be prevented by such an action of carrier particles, and therefore a developer having a longer lifetime is obtainable in comparison with the conventional developer. Consequently, the maintenance cycle or the cycle of replacing the developer can be elongated and the maintenance thereof and the like are simplified.
These objects and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the embodiments of the present invention when taken in conjunction with accompanying drawings.